Convocation (without reason)

First of all: Sorry to make this post in English, but unfortunately I don't speak french or dutch, and as a foreigner I would like to have some help in my situation.

I've received from the Police, a "Convocation" asking me to go to the Police Office. There are some days and hours that I can select to go there.

The letter says that I have to present myself with my ID + "permis" (which I believe it is my drive license). The letter has a space to explain WHY they are inviting me, but this part it is in blank!

I did not commit anything serious. I had some fines in the past, but was absolutely nothing like "driving 160 in a highway which the maximum is 120", and by the way, those fines were a long time ago...

Can somebody help me to explain what generally means this kind of letter?